Jordanians and Syrian refugees take part in a demonstration against Syria's President Bashar al-Assad, and to protest against the killing of at least 108 people in the Syrian town of Houla last Friday, outside the Syrian embassy in Amman, Jordan, Wednesday, May 30.

Ali Jarekji/Reuters

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Syria's Houla massacre last week was a war crime. This much is certain. After government shelling of Houla killed about 20 people there, a further 90 residents were hunted down in their homes and shops and then butchered, many of them children.

The massacre has shifted the international picture, with the mass expulsion of Syrian diplomats from Britain, the US, France, and six other countries, slightly tougher talk from the United Nations officials working with special envoy Kofi Annan, and a burst of outrage from politicians around the world. Who was responsible? The activists and their supporters insist it was the Syrian Army itself, but there is not yet any hard evidence, only indications. Analysts who know the region well expect that the murders were carried out by shabiha, pro-government militiamen who work in concert with the military.

The cui bono reasoning of some on the anti-imperial left, who suggest the massacre was carried out by President Bashar al-Assad's opponents (since it makes his regime look so bad), should be dismissed as the logical contortion that it is. The Assad family has killed and tortured tens of thousands to retain power down the decades.

The slaughter yesterday, and the discovery today, of 13 bound men who were executed near Deir al-Zhour punctuate a reality that has been long apparent: UN special envoy Kofi Annan's "peace plan" for Syria is a failure, with Mr. Assad and his allies determined to hold on to power and survive. Assad emphasized that to Russian television a few weeks ago, complaining of a propaganda war against him, denying massacres of civilians and concluding that "the main thing is to win in real life."

But what is to be done? This is where all certainty evaporates, and a landscape of imperfect, dangerous choices reveals itself.

Humanitarian interventionists insist the time has come for military pressure to be exerted from the outside and they're finding allies in major capitals. French President François Hollande fumed, "it is not possible to allow Bashar al-Assad's regime to massacre its own people," though he said military action would require UN Security Council approval.

Presidential hopeful Mitt Romney, who last July criticized President Barack Obama for supporting the NATO mission that helped drive Libya's Muammar Qaddafi from power (Mr. Romney fretted about "who’s going to own Libya if we get rid of the government there?”) wants arms shipments to the rebels and "more assertive measures to end the Assad regime." He blamed President Obama for "lack of leadership [that] has resulted in a policy of paralysis that has watched Assad slaughter 10,000 individuals."

He's not alone. The Washington Post's hawkish editorial page is on board too, sort of. In an editorial largely dedicated to ridiculing Mr. Annan's failed effort ("feckless," "one of the most costly diplomatic failures in UN history") it calls for Obama to do, well, something. The paper insists the time has come for US "leadership," but through what means, and exactly to where, it doesn't say.

The Post probably didn't intend to send a message with this vagueness, but the coyness on the specifics of what the US should be doing points to a reality that makes a foreign military intervention in Syria far more dangerous than the case of Libya. Syria's armed forces are better trained, led, and armed than Libya's were under Qaddafi, and they have held firm for over a year, whereas Qaddafi suffered major leadership defections from the moment the uprising broke out in February of last year.

More worrying still is the increasingly evident sectarian nature of some of the fighting. Libya has troubled divisions, but not much in the way of religious ones. Syria is home to Sunni Muslims, Christians, Alawites, Shiites, and ethnic Kurds.

Assad belongs to the Alawite sect, a Syrian minority that has benefited handsomely under his rule and his father's before him. The majority of the people fighting him are Sunni Muslims, and there is clear evidence that among them are Al Qaeda-style jihadis whose Manichean worldview frames the Alawites as apostates who deserve death for their beliefs. The country's Christians are fearful, well aware of the grim toll on their faith in Iraq. Tens of thousands of Iraqi Christians who were forced to flee their homeland still live among them.

This is not to say that the rebellion's grievances aren't real, or that the majority of its supporters want precisely what they say they do: To get out from under the yoke of Assad's Baath regime and to live in a more democratic country. With the events of the past year there, it's impossible to fault any Syrian who wants to take up arms against Assad. But the grievances of decades, and the fresh set of wounds inflicted on the population, create the conditions for the cycle of sectarian murder and counter-murder that extracted such horrific costs on Lebanon in the 1970s and 1980s, and in Iraq after the US-led invasion in 2003.

In Iraq, hundreds of thousands were tortured and murdered in communal violence that 100,000 US troops were largely powerless to stop. Foreign boots in Syria would be no more a guarantee of peace.

The people framed as the murderers today could be precisely the ones in desperate need of international protection tomorrow. And massacres after such an intervention will be partly on the hands of those foreign powers who got involved, no matter how well-intentioned. US officials and their counterparts in Europe are aware of these risks, as they are of political realities. Is it better to take action and risk being on the hook for future crimes that, after all, may not occur? Or is the safe play to look on aghast, let diplomacy and sanctions grind on, and hope a breakthrough – somehow, anyhow – will be found?

In Libya, the UN Security Council authorized NATO action. While from almost the moment the bombs started falling the US, France, the UK, and others clearly exceeded their mandate only to protect civilians from imminent harm, UN cover was the key to action. That was cover that Russia was willing to grant that time, choosing not to exercise its veto in the crucial vote. Later, the country was furious at how the UN mandate was used, and vowed there would be no repeats.

So far, Russian President Vladimir Putin has been unmoved by the killing. Russian Deputy Foreign Minister Gennady Gatilov said today his country opposed new Security Council resolutions on Syria and "it is essential to give the plan of Kofi Annan time to work." Time? The plan has failed, and a suggestion that there is going to be some magical about-face by Assad if only more time is given flies in the face of every available data point.

The news from Syria, the amateur video of fathers and mothers grieving over their children, and the stills of people clearly tortured to their deaths are almost too much to bear. The demands that "something must be done" are humane, natural, a healthy reaction to the horror. But determining what that "something" is, and doing it well, is the tough, cold work that policy makers have in front of them.